1. Field of the Invention
The present invention relates to an LC (liquid crystal) aligning agent using diamine having dendron side chains. More particularly, the present invention relates to a composition for an LC alignment film which employs diamine having dendron side chains to produce polyamic acid, followed by imidization. When the LC alignment film is applied to a liquid crystal display device, high heat resistance, high penetration in a visible ray range, excellent alignment, and a high voltage holding ratio are assured. Particularly, even though it contains a small amount of functional diamine, a high pretilt angle can be assured, thus providing easy control of the pretilt angle and improving a vertical aligning force.
2. Description of the Related Art
A liquid crystal display device is operated using the property in which the arrangement of liquid crystal changes depending on the application of external voltage due to the effect of an electric field and external light incident on the LCD is blocked or passed therethrough due to the change in arrangement. Functions of the LCD, such as light penetration, a response rate, a visual angle, and a contrast ratio, depend on the arrangement of liquid crystal molecules. Accordingly, it is very important to uniformly control the arrangement of the liquid crystal molecules.
The demand for high quality displays is continuously growing in accordance with the recent growth of an LCD market, and the demand for an alignment film having high productivity is growing as the trend toward a large area is rapidly increasing in the fabrication of the LCD. Hence, there is a need to develop an alignment film having various excellent properties, such as low defective fractions, excellent electrooptical properties, and high reliability, in a process of fabricating the LCD. That is to say, there is a need for an LC alignment film which has excellent printability, a stable pretilt angle, high voltage holding ratio, and sufficiently small voltage accumulation due to a direct current, and which is capable of being reworked.
The LC alignment film is a polymer substance which is formed between a transparent conductive film, such as indium tin oxide (ITO), and liquid crystals to achieve the uniform arrangement of liquid crystal molecules, that is, alignment, and is used as means for controlling the liquid crystals using a mechanical method, such as rubbing, or other methods.
In a conventional process of uniformly arranging, that is, aligning, the liquid crystals in the LCD, which is called a rubbing process, polymer, such as polyimide, is applied on a transparent conductive glass to form a polymer film, and the alignment film is rubbed using a roller which is wound by a rubbing cloth, such as nylon or rayon, and which rotates at a high rate to achieve alignment. Through the rubbing process, the liquid crystal molecules are aligned at a predetermined pretilt angle on a surface of the alignment film.
A conventional polyimide resin for an LC alignment film is produced by polycondensating monomers which are exemplified by aromatic acid dianhydride, such as pyromellitic dianhydride (PMDA) or biphthalic dianhydride (BPDA), and aromatic diamine, such as p-phenylene diamine (p-PDA), m-phenylene diamine (m-PDA), 4,4-methylene dianiline (MDA), 2,2-bis(aminophenyl)hexafluoropropane (HFDA), m-bis(aminophenoxy)diphenylsulfone (m-BAPS), p-bis(aminophenoxy)diphenylsulfone (p-BAPS), 4,4-bis(aminophenoxy)phenylpropane (BAPP), or 4,4-bis(aminophenoxy)phenylhexafluoropropane (HF-BAPP).
However, if only aromatic acid dianhydride and diamine are used as described above, thermal stability, resistance to chemicals, and mechanical properties are excellent, but, disadvantageously, transparency and solubility are reduced due to a charge transfer complex, and electrooptical characteristics are worsened. Effort has been made to avoid the disadvantages using an aliphatic cyclic acid dianhydride monomer or aliphatic cyclic diamine (Japanese Patent Laid-Open Publication No. 11-84391). Additionally, the use of a functional diamine having side chains or functional acid dianhydride having side chains has been introduced to increase the pretilt angle of liquid crystals and achieve stability (Japanese Patent Laid-Open Publication No. 06-136122). Furthermore, effort has been made to develop a vertical alignment film capable of being used in a vertically aligned mode (VA mode) in which liquid crystals are aligned perpendicular to a surface in the course of forming an LCD panel (U.S. Pat. No. 5,420,233).
However, the demand for high quality displays continuously grows in accordance with the recent growth of the LCD market, and the demand for an alignment film having high productivity grows as the trend toward a large area is rapidly increasing in the fabrication of the LCD. Hence, there is increased need to develop a high performance LC alignment film which has low defective fractions, excellent electrooptical properties, and high reliability, and is capable of meeting various requirements of the LCD in a process of fabricating the LCD.